In recent years, a considerable increase in the potential and actual uses of certain ceramics for structural, chemical, electrical and electronic applications has occurred because of the strength, corrosion resistance, electrical conductivity and high temperature stability characteristics of these materials. Major applications for ceramics include Si.sub.3 N.sub.4 /steel and Si.sub.3 N.sub.4 /Al joints in gas turbines and diesel engines, recuperators in heat exchangers, Si.sub.3 N.sub.4 /steel and Si.sub.3 N.sub.4 /Ti joints in fuel cells, and ZrO.sub.2 /steel joints in friction materials for bearings, bushings, brakes, clutches and other energy absorbing devices. Al.sub.2 O.sub.3, cordierite and mullite/steel, Si.sub.3 N.sub.4, and Al.sub.2 O.sub.3 --TiC/steel joints for materials in cutting tools and dies used in metal fabrication, SiC, Al.sub.2 O.sub.3 and BN/Al and steel joints for space and military applications such as rocket nozzles, armor, missile bearings, gun barrel liners and thermal protection barriers in space vehicles, and SiC/C, Al.sub.2 O.sub.3 /Si and Al.sub.2 O.sub.3 /Cu and Al joints in electronic devices are others.
To perform effectively and efficiently for many of these applications, ceramic components chosen often must coexist with or be bonded with metallic components and form the system as a whole. Integration of ceramic-ceramic/metal hybrid parts into existing engineering designs can significantly enhance the performance of components.
The joining of ceramic parts or ceramic-metal components, however, presents a number of problems. For example, ceramic materials may differ, and certainly ceramics and metals differ greatly in terms of modulus of elasticity, coefficient of thermal expansion, and thermal diffusivity. Accordingly, large thermally induced mechanical stresses are set up in the joint regions during bonding. In the past, this problem has been overcome only with limited success in using common techniques such as diffusion bonding, arc and oxyfuel fusion welding, brazing, soldering, and mechanical attachment. Thus, while diffusion bonding has proven useful for producing joints with good elevated temperature properties, the practicality of this method is limited since it frequently requires vacuum and/or hot pressing equipment. Alternatively, conventional fusion welding techniques create potentially critical conditions, since the material at the interface is superheated for a substantially long time, rises to accelerated reaction rates, and leads to extensive interdiffusion of species. This situation results in the formation of an entirely different microstructure with degraded mechanical and chemical properties. In general, fusion welding may only be considered for materials with low stress applications.
While soldering and brazing techniques are relatively simple to carry out and may be conducted at lower temperatures, the procedure requires elaborate surface preparation, and most importantly, the joints produced are limited to applications which do not involve high strength or high temperature.
Other techniques, such as mechanical interlocking or electron-beam welding, have their own peculiar drawbacks. For example, electron-beam welding requires the use of a vacuum chamber. Additionally, it cannot be used with ease for dielectrics because of charge buildup on the insulating ceramics.
Accordingly, there remains an important need for an improved method of ceramic-ceramic or ceramic-metal bonding. Most importantly, there is a need for such method which produces a bond suitable for high temperature applications. The invention addresses these needs.